An increasing number of electronic apparatus have been employing a touch panel (hereinafter the touch panel is referred to as TP), which enables the input of a certain specific signal corresponding to displayed contents by pressing the relevant place on the display screen with a pen or a finger. In the following, a conventional TP is described referring to the drawings. The drawings used in the present invention are shown expanded in the direction of panel thickness for the sake of easier understanding.
FIG. 10 is a cross sectional view of a conventional TP. Referring to FIG. 10, a first transparent substrate 1, or bottom substrate, is made of a glass, a plastic film, a plastic sheet or similar material. The first transparent substrate 1 is provided on an upper surface with a first transparent conductive layer 2 formed by sputtering indium tin oxide (ITO) or tin dioxide (SnO2). Provided on the first transparent conductive layer 2 are very small dot spacers 3 made of an insulating epoxy resin or similar material disposed at a certain specific pitch. A second transparent substrate 4, or upper substrate, made of a glass, a plastic film or a plastic sheet, is provided beneath its lower surface with a second transparent conductive layer 5 made of ITO, SnO2, or the like.
The first transparent substrate 1 and the second transparent substrate 4 are adhered together by an outer circumference part 6 so that the second transparent conductive layer 5 is opposed to and insulated from the first transparent conductive layer 2 with a certain specific clearance. An insulation pattern, an electrode pattern, a wiring pattern, a pattern of adhesive compound, or the like are provided in the outer circumference part 6.
The upper surface of the second transparent substrate 4 is covered with a hard coat layer 7 of pencil hardness of 3H, which is made of an acrylic resin in order to protect the upper surface from scratches or other possible damage caused when it is pressed with a pen or a finger.
A flexible circuit board 8 is fixed by an adhesive at one end with the outer circumference part 6 for delivering output signals generated at the first transparent conductive layer 2 and the second transparent conductive layer 5 to an external circuit (not shown), while another end is connected with the external circuit.
Next, operation of the above-configured conventional TP is described.
When a transparent substrate 4 is pressed with a finger or a pen at a specific position from above, the substrate bends at the position and along the surrounding area. This brings the first transparent conductive layer 2 and the second transparent conductive layer 5 into contact at the pressed position. The dot spacers 3 keep the transparent conductive layers separated so as to make no contact with each other in areas other than the pressed position.
The pressed position is detected by an external circuit via the flexible circuit board 8 in the form of a voltage ratio relative to a voltage applied to the transparent conductive layer 2 and the transparent conductive layer 5 at the position of contact.
Since the transparent conductive layers 2 and 5, made of ITO, SnO2or the like, have a slightly yellowish color, conventional TPs sometimes exhibit a yellowish color, which is one of the problems of the TPs.
Namely, when a conventional TP is attached on a monochrome liquid crystal display device, the display device is seen as slightly yellowish via the TP because the display device originally has a background color of gray-yellow or yellow-green. When a conventional TP is attached on a color display device such as a color liquid crystal display device, a color organic EL or the like, the color tone of the color display device sometimes changes, creating a sense of incompatibility in the display of white, among other colors. This impairs evaluation of the apparatus, and requests for improvement have been increasingly arising.
The yellow color can be numerically represented by the b* value of L*a*b* (hereinafter referred to as b* value) color specification in accordance with JIS (Japan Industrial Standard) Z 8729, which is equivalent with CIE (Commission Internationale de l'Ecrairage) LAB and CIE LUV. The larger the + value in b* value, the stronger the color yellow; whereas the larger the negative value, the stronger the color blue.
The glass with ITO layer normally has a b* value of +1 or greater, and the plastic film or sheet with ITO layer has a b* value of +1.5 or greater. So, a TP formed of a glass transparent substrate 1 and a plastic transparent substrate 4 has a b* value of +2.5 or greater, and a TP in which both of the transparent substrates 1 and 4 are plastic has a b* value of +3 or greater.
The present invention addresses the above problems, and provides a high quality TP which has a reduced yellowish appearance.